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quantitative and qualitative differences between PYTHIA8 and EPOS3 are found in the pT spectra when (for a given multiplicity class) the leading jet pT is increased. In addition, we showed that for low-multiplicity events the presence of jets can produce radial flow-like behavior, shown in Fig. 1 (right). The observed differences between the two event classes (low and high multiplicities) similar to those seen in the hadrochemistry measured in the jet and bulk regions in pp and Pb-Pb collisions by the ALICE collaboration. Motivated by our findings, we proposed to perform a similar analysis using experimental data from RHIC and the LHC.
Figure 1. Left: Average transverse momentum of identified charged hadrons (pions, kaons, and protons) at mid-rapidity as a function of the corrected track multiplicity in the range
|η|<2.4, for pp collisions at √s =13 TeV (filled symbols) and at lower energies (open symbols).
Lines are drawn to guide the eye. Right: Proton-to-pion particle ratio as a function of pT for low (black lines) and high (red lines) multiplicity event classes, and for different leading jet pT
intervals simulated by the EPOS3 Monte Carlo event generator.
Angular correlations. — Previous studies have shown that several mechanisms can play a role in producing collective-like behavior. It has been demonstrated that multi-parton interactions and color reconnection as implemented in PYTHIA MC event generator produce radial flow patterns via boosted color strings. Also, azimuthal correlations have been studied in A Multi-Phase Transport model (AMPT), where the ridge structure can be generated assuming incoherent elastic scattering of partons and the string melting mechanism. Besides, phenomenological studies (as described above) show that it is possible to find a subclass of low-multiplicity events where radial flow patterns arise, despite the fact that at very low multiplicity hydrodynamics cannot be applied and color reconnection effects are small.
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We have measured the two-particle angular correlations of charged particles in Pb-Pb collisions at √sNN = 2.76 TeV. The azimuthal angle () and pseudorapidity () difference of a trigger particle with high pT and an associated particle with lower pT are evaluated. In the distribution of these angles, jets manifest themselves as a peak around (, ) = (0, 0) and as an elongated structure in () at = 𝜋. Studying the centrality and pT dependence of the shape of the jet peak and comparing it to the shape in proton-proton collisions can provide insight on the interaction of jets with the quark-gluon plasma (QGP). The jet peak is found to broaden at low pT in Pb-Pb collisions towards central collisions (Fig. 2, left). It is also found to become asymmetric (broader in than in ). An unexpected depletion around (, ) = (0, 0) also develops at low pT. The comparison of the modification of the jet peak with the AMPT model shows that both effects are accompanied by large radial and longitudinal flow, suggesting that they arise as a consequence of the interaction of the jets with the flowing QGP.
Quantum correlations. — We have measured short-range two-particle correlation functions of identified hadrons in pp, p-Pb, and peripheral Pb-Pb collisions. The extracted radii of the particle emitting source (via Bose-Einstein correlations) are in the range 1-5 fm, reaching highest values for very high multiplicity p-Pb and Pb-Pb collisions. The pp and p-Pb source is elongated in the beam direction, while in the peripheral Pb-Pb case the source is symmetric.
The dependence of the radii on the multiplicity and kT factorizes and appears to be less sensitive to the type of the collision system and center-of-mass energy (Fig. 2, right). The observed similarities may point to a common critical hadron density reached in the collisions.
Heavy flavour production. — Heavy-flavour (beauty and charm) quarks are produced almost
exclusively in initial hard processes, and their yields remain largely unchanged throughout a heavy-ion reaction. Nevertheless, they interact with the nuclear matter in all the stages of its
Figure 2. Left: Width of the jet peak in the and directions from Pb-Pb and pp (rightmost points) collisions at √sNN = 2.76 TeV. Right: Cauchy-type radius parameters for pions from Bose-Einstein correlation analyses of various collision systems and center-of-mass energies as a function of the corrected track multiplicity in the range |η|<2.4, scaled to kT = 0.45 GeV/c with help of a specific parametrization.
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evolution. Thus, heavy quarks serve as ideal self-generated penetrating probes of the strongly interacting QGP. Jets containing heavy flavour hadrons are also sensitive to flavour-dependent fragmentation and gluon splitting. Recent heavy-flavour jet measurements by the ALICE experiment, with contributions from our group, provide strong constraints on theoretical models of heavy-flavour production and fragmentation.
Jet structure. — Non-trivial behavior of high multiplicity events in small systems have also been observed in the heavy-flavour sector. Recent analyses of pp and p-Pb collisions show a universal enhancement of heavy-flavour particles that is usually attributed to multiple parton interactions and higher gluon radiation associated with short distance production processes.
We have carried out extensive studies using MC event generators. We have given predictions for multiplicity-dependent jet structures, and proposed a way to validate the presence and extent of effects such as multiple-parton interactions or color reconnection, based on the detection of non-trivial jet shape modification in high multiplicity events. We proposed a way to use the multiplicity-dependent jet structures to experimentally differentiate between equally well-performing simulation tunes. We have also introduced a definition of a characteristic jet size measure that is independent of multiplicity. These studies can serve as a baseline for jet structure analyses in heavy-ion collisions as well as flavour-dependent studies.
New method for tracking of charged particles at high multiplicities. — We have developed a novel combination of established data analysis techniques for reconstructing charged particles in very high multiplicity collisions. It uses all information available while keeping competing choices open as long as possible. Suitable track candidates are selected by transforming measured hits to a track parameter space with help of templates. The highly connected network of track candidates and their corresponding hits is cut into very many subgraphs by removing a few of its vulnerable components, edges, and nodes. Finally, the hits distributed among the candidates by exploring a deterministic decision tree. A depth-limited search is performed maximizing the number of hits on tracks, and also the sum of track-fit quality measures.
Grants
NKFI K 109703: Consortional main: Hungary in the CMS experiment of the Large Hadron Collider (F. Siklér, 2013-2017)
Swiss National Science Foundation, SCOPES 152601: Preparation for and exploitation of the CMS data taking at the next LHC run (G. Dissertori ETHZ, 2014-2017)
International cooperation
ALICE, CMS, FOPI, NA49, and NA61 (CERN), PHENIX and STAR (RHIC)
Publications
Articles
1. Adam W et al. incl. Siklér F, Veszprémi V [674 authors]: P-Type Silicon Strip Sensors for the new CMS Tracker at HL-LHC. JOURNAL OF INSTRUMENTATION 12:(6) Paper P06018. 26 p. (2017)
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2. Aduszkiewicz A et al. incl. Fodor Z, László A, Márton K, Vesztergombi G [154 authors]:
Two-particle correlations in azimuthal angle and pseudorapidity in inelastic p + p interactions at the CERN Super Proton Synchrotron. EUR PHYS J C 77:(2) 59/1-15 p.
(2017)
3. Siklér F: Combination of various data analysis techniques for efficient track reconstruction in very high multiplicity events. EPJ WEB CONF 150: 00011/1-13 (2017) (CTD/WIT 2017 - Connecting The Dots/Intelligent Trackers, Orsay, France, 06-09 March 2017)
4. Siklér F: Femtoscopy with identified hadrons in pp, pPb, and PbPb collisions in CMS.
UNIVERSE 3:(4) 76/1-10 (2017)
5. Abbaneo D et al. incl. Bencze G, Endrőczi G [151 authors]: Overview of large area triple-GEM detectors for the CMS forward muon upgrade. NUCL INSTRUM METH A 845:
298-303 (2017)
6. Abbaneo D et al. incl. Bencze G, Endrőczi G [153 authors]: R&D on a new type of micropattern gaseous detector: The Fast Timing Micropattern detector. NUCL INSTRUM METH A 845: 313-317 (2017)
7. Ahmed W et al. incl. Bencze G, Endrőczi G [155 authors]: The Triple GEM Detector Control System for CMS forward muon spectrometer upgrade. J INSTRUM 12:
P02003/1-14 (2017)
8. Ortiz A, Bencédi G, Bello H: Revealing the source of the radial flow patterns in proton-proton collisions using hard probes. J PHYS G NUCL PARTIC 44:(6) 065001/1-14 (2017) 9. Zsigmond AJ: Z bozonok jelentõsége nehézion-ütközésekben (Significance of bosons
in heavy ion collisions, in Hungarian). FIZIKAI SZEMLE 67:(7-8) 251-254 (2017) Book chapter
10. Vértesi R: Production of quarkonia at RHIC. In: Gribov-85 Memorial Volume: Exploring Quantum Field Theory: Proceedings of the Memorial Workshop Devoted to the 85th Birthday of VN Gribov (Budapest, Hungary, 17-20 June 2015).Eds:: Dokshitzer Y, Lévai P, Nyíri J, Singapore: World Scientific Publishing, 2017 pp. 355-363
See also: R-I.1
CMS Collaboration
Due to the vast number of publications of the large collaborations in which the research group participated in 2015, here we list only a short selection of appearences in journals with the highest impact factor.
1. Khachatryan V et al. incl. Bencze G, Hajdu C, Hidas P, Horvath D, Sikler F, Veszpremi V, Vesztergombi G, Zsigmond AJ [2274 authors]: Search for Dark Matter and Supersymmetry with a Compressed Mass Spectrum in the Vector Boson Fusion Topology in Proton-Proton Collisions at √𝑠 = 8 TeV. PHYS REV LETT 118:(2) 021802/1-18 (2017)
2. Khachatryan V et al. incl. Bencze G, Hajdu C, Horvath D, Sikler F, Veszpremi V, Vesztergombi G, Zsigmond AJ [2272 authors]: Observation of Charge-Dependent Azimuthal Correlations in p-Pb Collisions and Its Implication for the Search for the Chiral Magnetic Effect. PHYS REV LETT 118:(12) 122301/1-17 (2017)
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3. Sirunyan AM et al. incl. Bencze G, Hajdu C, Horvath D, Sikler F, Veszpremi V, Vesztergombi G, Zsigmond AJ [2236 authors]: Search for Supersymmetry in p p Collisions at√𝑠 = 13 TeV in the Single-Lepton Final State Using the Sum of Masses of Large-Radius Jets. PHYS REV LETT 119: 151802/1-18 (2017)
4. Sirunyan AM et al. incl. Bencze G, Hajdu C, Horvath D, Sikler F, Veszpremi V, Vesztergombi G, Zsigmond AJ [2274 authors]: Relative Modification of Prompt ψ (2S) and J /ψ Yields from pp to PbPb Collisions at √𝑠 = 5.02 TeV. PHYS REV LETT 118:(16) Paper 162301. 17 p. (2017)
5. Sirunyan AM et al. incl. Bencze G, Hajdu C, Horvath D, Sikler F, Veszpremi V, Vesztergombi G, Zsigmond AJ [2239 authors]: Study of Jet Quenching with Z plus jet Correlations in Pb-Pb and pp Collisions at √𝑠 = 5.02 TeV. PHYS REV LETT 119:(8) 082301/1-18 (2017)
6. Sirunyan AM et al. incl. Bencze G, Hajdu C, Horvath D, Sikler F, Veszpremi V, Vesztergombi G, Zsigmond AJ [2231 authors]: Search for Low Mass Vector Resonances Decaying to Quark-Antiquark Pairs in Proton-Proton Collisions at √𝑠 = 13 TeV. PHYS REV LETT 119:(11) 111802/1-18 (2017)
7. Sirunyan AM et al. incl. Bencze G, Hajdu C, Horvath D, Sikler F, Veszpremi V, Vesztergombi G, Zsigmond AJ [2226 authors]: Search for Charged Higgs Bosons Produced via Vector Boson Fusion and Decaying into a Pair of W and Z Bosons Using pp Collisions at √𝑠 = 13 TeV. PHYS REV LETT 119:(14) 141802/1-18 (2017)
8. Sirunyan AM et al. incl. Bencze G, Hajdu C, Horvath D, Sikler F, Veszpremi V, Vesztergombi G, Zsigmond AJ [2196 authors]: Measurement of the B-+/- Meson Nuclear Modification Factor in Pb-Pb Collisions at √𝑠 = 5.02 TeV. PHYS REV LETT 119:(15) 152301/1-17 (2017)
9. Sirunyan AM et al. incl. Bencze G, Hajdu C, Horvath D, Sikler F, Veszpremi V, Vesztergombi G, Zsigmond AJ [2243 authors]: Search for Evidence of the Type-III Seesaw Mechanism in Multilepton Final States in Proton-Proton Collisions at √𝑠 = 13 TeV. PHYS REV LETT 119:(22) 221802/1-17 (2017)
10. Sirunyan AM et al. incl. Bencze G, Hajdu C, Horvath D, Sikler F, Veszpremi V [2254 authors]: Observation of Top Quark Production in Proton-Nucleus Collisions. PHYS REV LETT 119:(24) 242001.1-17 (2017)
11. Adam W et al. incl. Sikler F, Veszpremi V [696 authors]: Characterisation of irradiated thin silicon sensors for the CMS phase II pixel upgrade. EUR PHYS J C 77:(8) 567/1-13 (2017)
12. Khachatryan V et al. incl. Bencze G, Hajdu C, Hidas P, Horvath D, Sikler F, Veszpremi V, Vesztergombi G, Zsigmond AJ [2269 authors]: Measurement of the WZ production cross section in pp collisions at √s=7 and 8 TeV and search for anomalous triple gauge couplings at √𝑠 = 8TeV. EUR PHYS J C 77:(4) 236/1-29 (2017)
13. Khachatryan V et al. incl. Bencze G, Hajdu C, Horvath D, Sikler F, Veszpremi V, Vesztergombi G, Zsigmond AJ [2273 authors]: Suppression and azimuthal anisotropy of prompt and nonprompt J / ψ production in PbPb collisions at √𝑠 = 2.76TeV. EUR PHYS J C 77:(4) 252/1-33 (2017)
14. Khachatryan V et al. incl. Bencze G, Hajdu C, Hidas P, Horvath D, Sikler F, Veszpremi V, Vesztergombi G, Zsigmond AJ [2309 authors]: Measurements of the tt¯ production cross
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section in lepton+jets final states in pp collisions at 8 TeV and ratio of 8 to 7 TeV cross sections. EUR PHYS J C 77:(1) 15/1-27 (2017)
15. Khachatryan V et al. incl. Bencze G, Hajdu C, Hidas P, Horvath D, Sikler F, Veszpremi V, Vesztergombi G, Zsigmond AJ [2267 authors]: Measurement of the production cross section of a W boson in association with two b jets in pp collisions at √𝑠 = 8TeV. EUR PHYS J C 77:(2) 92/1-34 (2017)
16. Khachatryan V et al. incl. Bencze G, Hajdu C, Hidas P, Horvath D, Sikler F, Veszpremi V, Vesztergombi G, Zsigmond AJ [2278 authors]: Measurement of the t t ¯ production cross section using events in the e μ final state in pp collisions at √𝑠 = 13 TeV. EUR PHYS J C 77:(3) 172/1-22 (2017)
17. Khachatryan V et al. incl. Bencze G, Hajdu C, Hidas P, Horvath D, Sikler F, Veszpremi V, Vesztergombi G, Zsigmond AJ [2286 authors]: A search for new phenomena in pp collisions at √𝑠 = 13TeV in final states with missing transverse momentum and at least one jet using the αT variable. EUR PHYS J C 77:(5) 294/1-38 (2017)
18. Khachatryan V et al. incl. Bencze G, Hajdu C, Hidas P, Horvath D, Sikler F, Veszpremi V, Vesztergombi G, Zsigmond AJ [2282 authors]: Search for new phenomena with multiple charged leptons in proton–proton collisions at √𝑠 = 13TeV. EUR PHYS J C 77:(9) 635/1-29 (2017)
19. Khachatryan V et al. incl. Bencze G, Hajdu C, Hidas P, Horvath D, Sikler F, Veszpremi V, Vesztergombi G, Zsigmond AJ [2296 authors]: Measurements of the associated production of a Z boson and b jets in pp collisions at √𝑠 = 8TeV. EUR PHYS J C 77:(11) 751/1-34 (2017)
20. Sirunyan AM et al. incl. Bencze G, Hajdu C, Horvath D, Sikler F, Veszpremi V, Vesztergombi G, Zsigmond AJ [2236 authors]: Measurement of prompt and nonprompt J / ψ production in p p and p Pb collisions at √𝑠 = 5.02TeV. EUR PHYS J C 77:(4) 269/1-27 (2017)
NA61/SHINE Collaboration
1. Aduszkiewicz A et al. incl. Fodor Z, László A, Vesztergombi G [154 authors]: Two-particle correlations in azimuthal angle and pseudorapidity in inelastic p + p interactions at the CERN Super Proton Synchrotron. EUR PHYS J C 77:(2) 59/1-15 (2017)
2. Aduszkiewicz A et al. incl. Fodor Z, László A, Márton K [145 authors]: Measurement of meson resonance production in π-+ C interactions at SPS energies. EUR PHYS J C 77:(9) 626/1-26 (2017)
3. Aduszkiewicz A et al. incl. Fodor Z, László A, Márton K, Vesztergombi G [143 authors]:
Measurements of π± , K± , p and 𝑝̅ spectra in proton-proton interactions at 20, 31, 40, 80 and 158 GeV/c with the NA61/SHINE spectrometer at the CERN SPS. EUR PHYS J C 77:(10) 671/1-41 (2017)
4. Aduszkiewicz A et al. incl. László A, Márton K [140 authors]: NA61/SHINE Collaboration.
NUCL PHYS A 967: pp. 994-995. (2017) (26th International Conference on Ultrarelativistic Nucleus-Nucleus Collissions (Quark Matter). Chicago, USA, 05. February 2017 – 11.
February 2017)
5. Aduszkiewicz A for the NA61/SHINE Collaboration: Recent results from NA61/SHINE.
NUCL PHYS A 967: 35-42 (2017) (26th International Conference on Ultrarelativistic
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Nucleus-Nucleus Collissions (Quark Matter). Chicago, USA, 05. February 2017 – 11.
February 2017)
STAR Collaboration
1. Vértesi R and STAR Collaboration: Latest results on Υ production in heavy-ion collisions from the STAR experiment. J PHYS Pokol
CONF SER 832: 012025-1-4 (2017) (7th Workshop on Young Scientists on the Physics of Ultra-RelativisticNucleus-Nucleus Collisions (Hot Quarks). Southbridge, USA, 12 September 2016 – 17 September 2016)
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